v8.0 module release (#22975)

https://mattermost.atlassian.net/browse/MM-52079

```release-note
We upgrade the module version to 8.0. The new module path is github.com/mattermost-server/server/v8.
```


Co-authored-by: Doug Lauder <wiggin77@warpmail.net>

server/model/plugin_cluster_event.go

@@ -0,0 +1,28 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.txt for license information.
+
+package model
+
+const (
+	PluginClusterEventSendTypeReliable   = ClusterSendReliable
+	PluginClusterEventSendTypeBestEffort = ClusterSendBestEffort
+)
+
+// PluginClusterEvent is used to allow intra-cluster plugin communication.
+type PluginClusterEvent struct {
+	// Id is the unique identifier for the event.
+	Id string
+	// Data is the event payload.
+	Data []byte
+}
+
+// PluginClusterEventSendOptions defines some properties that apply when sending
+// plugin events across a cluster.
+type PluginClusterEventSendOptions struct {
+	// SendType defines the type of communication channel used to send the event.
+	SendType string
+	// TargetId identifies the cluster node to which the event should be sent.
+	// It should match the cluster id of the receiving instance.
+	// If empty, the event gets broadcasted to all other nodes.
+	TargetId string
+}